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Trees in Australia's tropical rainforests have achieved a global first by transitioning from serving as a CO2 absorber to becoming a source of emissions, driven by increasingly extreme temperatures and arid environments.

Critical Change Identified

This significant change, which impacts the stems and limbs of the trees but does not include the root systems, began approximately a quarter-century back, according to recent research.

Forests typically absorb carbon as they develop and release it upon decay and death. Overall, tropical forests are considered carbon sinks – absorbing more CO2 than they release – and this uptake is expected to grow with higher CO2 levels.

However, nearly 50 years of data collected from tropical forests across Queensland has revealed that this essential carbon sink could be under threat.

Study Insights

Roughly 25 years ago, tree stems and limbs in these forests turned into a carbon source, with more trees dying and insufficient new growth, as the study indicates.

“It’s the first tropical forest of its kind to show this symptom of change,” commented the principal researcher.

“We know that the moist tropics in Australia occupy a slightly warmer, drier climate than tropical forests on other continents, and therefore it could act as a coming example for what tropical forests will experience in global regions.”

Worldwide Consequences

A study contributor mentioned that it remains to be seen whether Australia’s tropical forests are a harbinger for other tropical forests globally, and further research are required.

But should that be the case, the results could have major consequences for global climate models, carbon budgets, and environmental regulations.

“This paper is the first time that this tipping point of a transition from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not just for one year, but for 20 years,” stated an authority on climate science.

On a global scale, the share of carbon dioxide taken in by forests, trees, and plants has been relatively constant over the last 20 to 30 years, which was assumed to continue under many climate models and strategies.

But if similar shifts – from sink to source – were observed in other rainforests, climate projections may understate heating trends in the coming years. “Which is bad news,” it was noted.

Continued Function

Even though the equilibrium between gains and losses had shifted, these forests were still serving a vital function in soaking up CO2. But their reduced capacity to take in additional CO2 would make emissions cuts “a lot harder”, and necessitate an even more rapid transition away from fossil fuels.

Research Approach

This study drew on a distinct collection of forest data starting from 1971, including records monitoring roughly 11,000 trees across 20 forest sites. It focused on the carbon stored in trunks and branches, but excluded the changes in soil and roots.

An additional expert emphasized the importance of gathering and preserving extended datasets.

“We thought the forest would be able to absorb additional CO2 because [CO2] is increasing. But looking at these long term empirical datasets, we find that is not the case – it allows us to compare models with actual data and improve comprehension of how these ecosystems work.”